Implantable medical devices are sometimes used for delivery of a therapeutic agent, such as a drug, to an organ or tissue in the body. It is hoped that these devices may deliver agents to a wide variety of bodily systems to provide a wide variety of treatments.
One implantable medical device which has been used for local delivery of therapeutic agents is the coronary stent. Coronary stents are typically introduced percutaneously, and transported transluminally until positioned at a desired location. These devices are then expanded either mechanically, such as by the expansion of a mandrel or balloon positioned inside the device, or expand themselves by releasing stored energy upon actuation within the body. Once expanded within the lumen, these devices, called stents, become encapsulated within the body tissue and remain a permanent implant.
Of the many problems that may be addressed through stent-based local delivery of therapeutic agents, one of the most important is restenosis. Restenosis is a major complication that can arise following vascular interventions such as angioplasty and the implantation of stents. Simply defined, restenosis is a wound healing process that reduces the vessel lumen diameter by extracellular matrix deposition, neointimal hyperplasia, and vascular smooth muscle cell proliferation, and which may ultimately result in renarrowing or even reocclusion of the lumen. Despite the introduction of improved surgical techniques, devices, and pharmaceutical agents, the overall restenosis rate is still reported in the range of 25% to 50% within six to twelve months after an angioplasty procedure. To treat this condition, additional revascularization procedures are frequently required, thereby increasing trauma and risk to the patient.
One of the techniques under development to address the problem of restenosis is the use of surface coatings of various therapeutic agents on stents. U.S. Pat. No. 5,716,981, for example, discloses a stent that is surface-coated with a composition comprising a polymer carrier and paclitaxel (a well-known compound that is commonly used in the treatment of cancerous tumors). Known surface coatings, however, can provide little actual control over the release kinetics of therapeutic agents. These coatings are generally very thin, typically 5 to 8 microns deep. The surface area of the stent, by comparison is very large, so that the entire volume of the therapeutic agent has a very short diffusion path to discharge into the surrounding tissue. The ability to shape the release profiles from such systems is severely limited.
Accordingly, it would be desirable to provide a therapeutic agent delivery device with the ability to program the release kinetics to the particular agent, administration period, and release rate desired.